[Bug target/50910] New: [avr] inefficient division by 2

[Bug target/50910] New: [avr] inefficient division by 2

[gjl at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50910

             Bug #: 50910
           Summary: [avr] inefficient division by 2
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: gjl@gcc.gnu.org
        ReportedBy: gjl@gcc.gnu.org
                CC: eric.weddington@atmel.com
            Target: avr


The following source

char c;

void bar_c (int x)
{
    c = x ? x/2 : c+1;
}

reveals two issues

== 1 == 

Compiled with

$ avr-gcc -S -Os

there is a call to libgcc's division routine. RTX costs of division has to be
checked and costs of loading the constant added to the costs of division.

bar_c:
    sbiw r24,0
    breq .L2
    ldi r22,lo8(2)
    clr r23
    rcall __divmodhi4
    rjmp .L3
.L2:
    lds r22,c
    subi r22,lo8(-(1))
.L3:
    sts c,r22
    ret

== 1 == 

Compiled with

$ avr-gcc -S -O2

the code is happily jumping around to L7 and then back to L3.

bar_c:
    sbiw r24,0
    brne .L6
    lds r24,c
    subi r24,lo8(-(1))
    sts c,r24
    ret
.L6:
    sbrc r25,7
    rjmp .L7
.L3:
    asr r25
    ror r24
    sts c,r24
    ret
.L7:
    adiw r24,1
    rjmp .L3

Presumably, the cause is 
    #define BRANCH_COST 0
in avr.h, and the sequence could be instead:

    ...
.L6:
    sbrc r25,7
    adiw r24,1
.L3:
    asr r25
    ror r24
    sts c,r24
    ret

[Bug target/50910] [avr] inefficient division by 2

[gjl at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50910

Georg-Johann Lay <gjl at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Priority|P3                          |P4
             Status|UNCONFIRMED                 |ASSIGNED
   Last reconfirmed|                            |2011-10-29
   Target Milestone|---                         |4.7.0
     Ever Confirmed|0                           |1
      Known to fail|                            |4.3.3, 4.5.2, 4.6.2, 4.7.0
           Severity|normal                      |minor

[Bug target/50910] [avr] inefficient division by 2

[gjl at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50910

--- Comment #1 from Georg-Johann Lay <gjl at gcc dot gnu.org> 2011-11-01 14:10:23 UTC ---
Author: gjl
Date: Tue Nov  1 14:10:13 2011
New Revision: 180739

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=180739
Log:
    PR target/50910
    * config/avr/avr.opt (-mbranch-cost=): New option.
    * config/avr/avr.h (BRANCH_COST): Define to avr_branch_cost.
    * config/avr/avr.c (avr_rtx_costs_1): Adjust [U]DIV/[U]MOD costs.
    * config/avr/avr.md (*addqi3.lt0, *addhi3.lt0, *addsi3.lt0): New insns.
    (*addhi3_zero_extend1): Remov % in constraint of operand 1.
    (*addhi3.sign_extend1, *subhi3.sign_extend2): New insns.


Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/config/avr/avr.c
    trunk/gcc/config/avr/avr.h
    trunk/gcc/config/avr/avr.md
    trunk/gcc/config/avr/avr.opt

[Bug target/50910] [avr] inefficient division by 2

[gjl at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50910

Georg-Johann Lay <gjl at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|                            |FIXED

--- Comment #2 from Georg-Johann Lay <gjl at gcc dot gnu.org> 2011-11-01 14:11:19 UTC ---
Fixed in 4.7.0